U.S. patent application number 10/579647 was filed with the patent office on 2009-05-07 for thin-film heating element.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS N.V.. Invention is credited to Roel Rethmeier, Pieter Johannes Werkman.
Application Number | 20090114639 10/579647 |
Document ID | / |
Family ID | 34610071 |
Filed Date | 2009-05-07 |
United States Patent
Application |
20090114639 |
Kind Code |
A1 |
Werkman; Pieter Johannes ;
et al. |
May 7, 2009 |
THIN-FILM HEATING ELEMENT
Abstract
The invention relates to a heating element comprising an
aluminum substrate, an electrically insulating layer based on a
sol-gel precursor, and an electrically resistive layer with a
thickness smaller than 2 .mu.m. The features of this heating
element solve the problem of the crack formation due to a mismatch
of thermal expansion coefficient of the aluminum substrate and the
resistive layer. Also disclosed is an electrical domestic appliance
comprising the heating element of the invention.
Inventors: |
Werkman; Pieter Johannes;
(Singapore, SG) ; Rethmeier; Roel; (Drachten,
NL) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
PO BOX 3001
BRIARCLIFF MANOR
NY
10510-8001
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS
N.V.
Eindhoven
NL
|
Family ID: |
34610071 |
Appl. No.: |
10/579647 |
Filed: |
November 11, 2004 |
PCT Filed: |
November 11, 2004 |
PCT NO: |
PCT/IB2004/052382 |
371 Date: |
January 23, 2009 |
Current U.S.
Class: |
219/543 ;
29/611 |
Current CPC
Class: |
H05B 3/262 20130101;
D06F 75/24 20130101; Y10T 29/49083 20150115 |
Class at
Publication: |
219/543 ;
29/611 |
International
Class: |
H05B 3/26 20060101
H05B003/26; H05B 3/16 20060101 H05B003/16 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 20, 2003 |
EP |
03078688.3 |
Claims
1. A film heating element, at least comprising an aluminum
substrate, an electrically insulating layer which is based on a
sol-gel precursor, and an electrically resistive layer with a
thickness smaller than 2 .mu.m.
2. A film heating element as claimed in claim 1, wherein the
electrically resistive layer comprises an inorganic material.
3. A film heating element as claimed in claim 1, wherein the
sol-gel precursor is a hybrid sol-gel precursor comprising an
organosilane compound.
4. A heating element as claimed in claim 3, characterized in that
the organosilane compound comprises methyl-trimethoxysilane or
methyl-triethoxysilane.
5. A heating element as claimed in claim 1, wherein the heating
element further comprises a conductive layer.
6. An electrical domestic appliance comprising at least a heating
element in accordance with claim 1.
7. An electrical domestic appliance according to claim 6,
characterized in that the electrical domestic appliance comprises a
(steam) iron, a hair dryer, a hair styler, a steamer and a steam
cleaner, a garment cleaner, a heated ironing board, a facial
steamer, a kettle, a pressurized boiler for system irons and
cleaners, a coffee maker, a deep-fat fryer, a rice cooker, a
sterilizer, a hot plate, a hot-pot, a grill, a space heater, a
waffle iron, a toaster, an oven, or a water flow heater.
8. A method of manufacturing a heating element according to claim
1, at least comprising the steps of: providing an aluminum
substrate; applying an electrically insulating layer on said
substrate; and applying a resistive layer on top of the
electrically insulating layer, characterized in that the
electrically insulating layer is obtained by means of a sol-gel
process and the resistive layer has a thickness smaller than 2
.mu.m.
Description
[0001] The present invention relates to a film heating element
comprising an aluminum substrate, an electrically insulating layer,
and an electrically resistive layer, as well as to an electrical
domestic appliance comprising such a heating element.
[0002] In general, a film heating element consists of two
functional layers applied on a substrate, namely, an electrically
insulating layer and an electrically resistive layer. Heat is
generated by flow of an electrical current through the resistive
layer. The function of the insulating layer is to isolate the
heat-generating resistive layer from the metal substrate, which may
be directly accessible from the outside.
[0003] The resistive layer can be electrically contacted with a
supply voltage via highly conductive tracks. These conductive
tracks are generally patterned.
[0004] Flat-film heating elements can be roughly divided into two
main categories, namely thick-film heating elements and thin-film
heating elements.
[0005] The distinction between these two categories concerns the
thickness of the resistive layer. In thick-film heating elements,
the resistive layer has a thickness exceeding 2 .mu.m. These films
are mainly prepared by means of screen-printing techniques. In
thin-film heating elements, the resistive layer has a thickness
smaller than 2 .mu.m.
[0006] These films are mainly prepared by means of evaporation
techniques or via pyrolysis of precursor solutions.
[0007] A thin-film heating element is known from U.S. Pat. No.
4,889,974. Said patent discloses a thin-film heating element
prepared by means of a wet-chemical process. This thin-film heating
element consists of a resistive layer applied directly on an
isolating substrate such as a hard glass substrate, a quartz glass
substrate, or a ceramic substrate. An SnO.sub.2 film doped with
acceptor- and donor-forming elements is described as a resistive
layer. The films are manufactured from a solution by means of a
spray pyrolysis process followed by curing at 600.degree. C.
[0008] A number of patents disclose thin-film heaters on
electrically conductive substrates, e.g. steel. An insulating layer
(e.g. polymer, enamel, etc.) is applied on these electrically
conductive substrates in order to insulate the resistive layer from
the substrate. A thin resistive layer is applied on top of these
insulating layers.
[0009] However, until recently no thin-film heaters on aluminum or
aluminum alloy substrates have been reported. Aluminum and its
alloys have a relatively high coefficient of expansion (22-26
ppm/K) compared to the insulating layers used for steel substrates
which are in most cases enamel-based insulators. Insulating layers
commonly used for steel substrates cannot be used for aluminum
(alloy) substrates. Mismatched thermal expansion coefficients
result in cracking of the film when the heating element is exposed
to temperature cycles. Furthermore, in order to apply these
insulators, the precursors are applied on a suitable substrate,
after which the precursor has to be cured at high temperatures
above 650.degree. C. in order to obtain a suitable insulating
layer. These high curing temperatures exceed or are near to the
melting temperature of aluminum (660.degree. C.) and its alloys.
Therefore, these materials are not suitable as electrically
insulating layers for aluminum substrates
[0010] EP-A-0891118 discloses a thin-film heater in which a ceramic
layer is used as an insulating layer for an aluminum substrate.
However, the difference in expansion coefficients between the
ceramic insulator layer and the aluminum is bridged in this patent
in that the heating element is first provided on a stainless steel
plate, after which the stainless steel plate is glued to an
aluminum plate with e.g. a silicone-based glue.
[0011] It is an object of the present invention to provide a
heating element of the preamble suitable for an aluminum substrate
in which no cracks are formed when the element is subjected to
temperature cycles. Where the term aluminum is used, it comprises
aluminum, anodized aluminum, and alloys of aluminum. Furthermore,
the present invention aims to provide an electrical domestic
appliance including such a heating element, as well as to a method
of manufacturing said heating element.
[0012] These and other objects of the invention are achieved by a
film heating element, at least comprising an aluminum substrate, an
electrically insulating layer which is based on a sol-gel
precursor, and an electrically resistive layer with a thickness
smaller than 2 .mu.m.
[0013] A heating element according to the invention has several
advantages. First of all no crack formation is observed when the
heating element is exposed to temperature cycles between 20 and
300.degree. C.
[0014] Furthermore, the heating element is suitable for high-power
applications, with a power density of 20 W/cm.sup.2 or higher at a
substrate temperature of 300.degree. C.
[0015] The film heating element according to the invention
comprises an electrically resistive layer with a thickness smaller
than 2 .mu.m. This resistive layer preferably comprises a metal, a
metal oxide, or a doped metal oxide. A suitable metal is aluminum.
Suitable metal oxides are tin oxide, indium-tin oxide (ITO).
Suitable doped metal oxides are fluorine or aluminum-doped zinc
oxide, or tin oxides doped with fluorine or antimony.
[0016] It was surprisingly found that, although e.g. ITO has a
thermal expansion coefficient of about 4 ppm/K compared to about 23
ppm/K for aluminum, no crack formation was observed when the
heating element of the invention was exposed to repeated
temperature cycles between 20 and 300.degree. C.
[0017] The resistive layer may be applied to the insulating layer
by means of (atmospheric) chemical vapor deposition ((A) CVD),
physical vapor deposition (PVD), magnetron sputtering, thermal
spraying, or wet-chemical techniques.
[0018] The resistive layer preferably consists of an inorganic
material. Suitable inorganic materials are a metal, a metal oxide,
and a doped metal oxide. A suitable metal is aluminum. Suitable
metal oxides are tin oxide, indium-tin oxide (ITO). Suitable doped
metal oxides are fluorine or aluminum-doped zinc oxide, or tin
oxides doped with fluorine or antimony. Resistive layers of an
inorganic material do not risk the formation of a carbonized
conductive track.
[0019] The heating element of the invention further comprises an
electrically insulating layer that is based on a sol-gel
precursor.
[0020] The application of an electrically insulating layer based on
a sol-gel precursor provides several advantages. First of all, the
sol-gel precursor based layer shows excellent electrical insulating
properties. The carbon content of a sol-gel precursor based
material is sufficiently low to prevent the formation of a
carbonized conductive track in case of failure of the heating,
thereby providing a safe heating element. Also, sol-gel materials
have a high thermal conductivity which is in the order of magnitude
of 0.1-2 W/m/.degree.K. Furthermore, sol-gel precursors can be
processed at temperatures below 400.degree. C., which makes this
material suitable to be applied directly to aluminum substrates.
Due to the lower curing temperature of the hybrid sol-gel
precursor, the mechanical properties of the aluminum will be
maintained. The sol-gel precursor is preferably applied on an
anodized aluminum substrate, to ensure good adhesion of the sol-gel
layer.
[0021] Although the sol-gel insulating layer is especially suitable
for application on aluminum substrates, other substrates which are
conventionally used for heating elements and which are compatible
with the final utility may also be used. Said substrates may
include, for example, stainless steel, enameled steel, or copper.
The substrate may be in the form of a flat plate, a tube, or any
other configuration that is compatible with the final utility.
[0022] Preferably, the sol-gel precursor is a hybrid sol-gel
precursor comprising an organosilane compound.
[0023] A preferred silane is a silane that forms a hybrid sol-gel
precursor. A hybrid sol-gel precursor comprising an organosilane
compound is understood to be a compound comprising silicon, which
is bonded to at least one non-hydrolysable organic group and 2 or 3
hydrolyzable organic groups.
[0024] In an advantageous embodiment, the sol-gel material may also
comprise silica particles, in particular colloidal silica
particles.
[0025] In particular, the hybrid sol-gel precursor comprises an
organosilane compound from the group of alkyl-alkoxysilanes.
[0026] Preferably, the hybrid sol-gel precursor comprises
methyl-trimethoxysilane (MTMS) and/or methyl-triethoxysilane
(MTES). An advantage of the heating element of the invention based
on the hybrid sol-gel system is a relatively high power density,
and optimized thermal expansion coefficient values for
aluminum.
[0027] Hybrid sol-gel precursors such as MTMS and MTES are known to
have an excellent temperature stability up to at least 450.degree.
C. Moreover, MTMS has been shown to prevent silver oxidation and
subsequent migration effectively. The carbon content of these
materials is still low, so carbonized conductive tracks across the
insulating layer will not form after failure, making a safe heating
element. The maximum layer thickness of coatings made from hybrid
precursors is relatively high, compared to the maximum layer
thickness of coatings made from non-hybrid sol-gel materials.
Therefore, the layers can be deposited in one or at most two steps
without intermediate curing.
[0028] Advantageously, the electrically insulating layer comprises
non-conductive particles.
[0029] A fraction of said non-conductive particles preferably has a
flake-like shape and a longest dimension of 2-500 .mu.m, preferably
from 2 to 150 .mu.m, and more preferably from 5 to 60 .mu.m. These
flake-like non-conductive particles are based on oxides such as,
for example, mica or clay, and/or surface-modified mica or clay
particles with a coating of titanium dioxide, aluminum oxide,
and/or silicon dioxide. The flake-like material content in the
insulating layer should be less than 20 vol %, preferably less than
15 vol %, and more preferably less than 4-10 vol %. An advantage of
such anisotropic particles is that their presence prevents the
formation of cracks in the electrically insulating layer after
frequent heating up and cooling down of the element.
[0030] In the preferred embodiment, the non-conductive particles
are present in colloidal form. Examples thereof are oxides like
aluminum oxide and silicon dioxide. Preferably, the aluminum oxide
content in the insulating layer should be less than 40 vol %,
preferably less than 20 vol %, and more preferably 10-15 vol %. As
for the silicon dioxide content in the insulating layer, it should
advantageously be less than 50 vol %, preferably less than 35 vol
%, and more preferably less than 15-25 vol %.
[0031] If an insulating layer is based on MTMS or MTES filled with
particles, including anisotropic particles, a layer thickness of
just 50 .mu.m can withstand 5000V. This relatively small layer
thickness allows the temperature difference across the thickness of
the resistive layer to be fairly low, which allows for a much lower
temperature of the heating resistive layer for obtaining a certain
temperature of the aluminum substrate. For this reason said thin
layers are advantageously used. The layers may be applied by any
wet-chemical application method, preferably spray coating or
screen-printing followed by a curing step.
[0032] The heating element according to the invention may further
comprise an electrically conductive layer. The electrically
conductive layer in the heating element of the invention comprises
a layer with a relatively low ohmic resistance with respect to the
electrically resistive layer and acts as a contacting layer between
the heat-generating resistive layer and an external power
source.
[0033] The conductive layer may consist of a metal, e.g. aluminum,
or of a hybrid material such as PI/Ag, or a sol-gel/Ag paste. The
conductive layer may be applied by means of (A)CDV, PVD, magnetron
sputtering, thermal spraying, and wet-chemical or screen printing
techniques.
[0034] The preferred technique for applying the conductive tracks
is screen printing. Commercially available metal powders may be
used for the conductive track. It is preferred to use silver or
silver alloy particles
[0035] Other metals and semiconductors may be used in making
conductive layers for the application, provided they have a
sufficiently high temperature stability in the sol-gel matrix. The
use of MTMS or MTES precursors reduces the rate of oxidation of
silver and graphite particles at high temperatures of the heating
element. In that respect it has been noted that graphite in an MTES
derived matrix has shown a stability of more than 600 hours at
320.degree. C.
[0036] To make the formulations screen-printable, a cellulose
derivative may be added to the particle-containing, hydrolyzed MTMS
or MTES solution. Hydroxyl-propylmethyl cellulose is preferably
used as the cellulose material. Finally, a solvent with a high
boiling point is added to prevent drying of the ink and subsequent
clogging of the screen. Butoxyethanol was found to be a suitable
choice, but other polar solvents, preferably alcohols, are also
found appropriate.
[0037] Optionally, the element may be covered with a protective
topcoat layer. This topcoat layer mainly serves as a protective
layer against mechanical damage during handling of the element.
With the use of, for instance, silica-filled hybrid sol-gel
solution, for example based on MTMS, a screen-printable formulation
can be easily made. The applied topcoat layer may be co-cured with
the conductive layer and the resistive layer.
[0038] The invention further relates to an electrical domestic
appliance comprising at least the heating element of the invention.
Heating elements of the present invention are very suitable for
heating elements in laundry irons, especially for the controlled
formation of steam, for which high power densities are required.
However, the heating elements are also very suitable for other
domestic applications like hair dryers, hair stylers, steamers and
steam cleaners, garment cleaners, heated ironing boards, facial
steamers, kettles, pressurized boilers for system irons and
cleaners, coffee makers, deep-fat fryers, rice cookers,
sterilizers, hot plates, hot-pots, grills, space heaters, waffle
irons, toasters, ovens, or water flow heaters.
[0039] The invention also relates to a method of manufacturing a
heating element according to the invention, at least comprising the
steps of: providing an aluminum substrate; applying an electrically
insulating layer on said substrate; and applying a resistive layer
on top of the electrically insulating layer, characterized in that
the electrically insulating layer is obtained by means of a sol-gel
process and the resistive layer has a thickness smaller than 2
.mu.m. In particular, the sol-gel process at least comprises the
step of mixing an organosilane compound with water.
[0040] The invention will be further elucidated in the following
manufacturing example.
EXAMPLE
[0041] A 200 nm thin layer (72*64 mm) of ITO (90 wt %
In.sub.2O.sub.3, 10 wt % SnO.sub.2 purity more than 99.99%) was
applied by means of DC magnetron sputtering in an argon/oxygen
atmosphere with a Leybold Z650 Batch system (starting initial
pressure below 4.0*10.sup.-6 mBar, deposition speed 20 nm/min) onto
a 50 .mu.m thick insulating layer based on a sol-gel precursor on
an aluminum substrate. Conductive layers (PI/Ag-based paste, PM437
by Acheson) of about 10 .mu.m thick were applied by means of screen
printing. After drying for 30 minutes at 80.degree. C., the
conductive layer was cured for 30 minutes at 375.degree. C. in an
air atmosphere. The resulting resistance is about 36 .OMEGA. with a
surface resistance of 0.27 .OMEGA./.quadrature. (for a 25.5 .mu.m
thick layer)
[0042] After application of a voltage, the resulting heating
element operates with a power density of 20 W/cm.sup.2 at a
substrate temperature setting of 240.degree. C.
* * * * *